1. Field of the Invention
The present invention relates to a variable magnification optical system and an image pickup apparatus using the same and, more specifically, to an optical arrangement suitable for use in a video camera, a still video camera, a copying machine and the like which are arranged to realize variation of magnification by using an optical unit having decentered reflecting surfaces, as a magnification varying optical unit.
2. Description of Related Art
It is known that an optical system of the type which is composed of only refracting lenses has been provided as a variable magnification optical system. In such a conventional optical system, refracting lenses each having a spheric surface or aspheric surface of rotational symmetry are rotationally symmetrically arranged with respect to the optical axis.
In addition, various photographing optical systems using reflecting surfaces such as concave mirrors or convex mirrors have heretofore been proposed, and an optical system using both a reflecting system and a refracting system is also well known as a catadioptric system.
FIG. 37 is a schematic view of a so-called mirror optical system which is composed of one concave mirror and one convex mirror. In the mirror optical system shown in FIG. 37, an object light beam 104 from an object is reflected by a concave mirror 101 and travels toward an object side while being converged, and after having been reflected by a convex mirror 102 and having been refracted by a lens 110, the object light beam 104 forms an image of the object on an image plane 103.
This mirror optical system is based on the construction of a so-called Cassegrainian reflecting telescope, and is intended to reduce the entire length of the optical system by bending, by using the two opposed reflecting mirrors, the optical path of a telephoto lens system which is composed of refracting lenses and has an entire large length.
For similar reasons, in the field of an objective lens system which constitutes part of a telescope as well, in addition to the Cassegrainian type, various other types which are arranged to reduce the entire length of an optical system by using a plurality of reflecting mirrors have been known.
As is apparent from the above description, it has heretofore been proposed to provide a compact mirror optical system by efficiently bending an optical path by using reflecting mirrors in place of lenses which are commonly used in a photographing lens whose entire length is large.
However, in general, the mirror optical system, such as the Cassegrainian reflecting telescope, has the problem that part of an object ray is blocked by the convex mirror 102. This problem is due to the fact that the convex mirror 102 is placed in the area through which the object light beam 104 passes.
To solve the problem, it has been proposed to provide a mirror optical system which employs decentered reflecting mirrors to prevent a portion of the optical system from blocking the area through which the object light beam 104 passes, i.e., to separate a principal ray of the object light beam 104 from an optical axis 105.
FIG. 38 is a schematic view of the mirror optical system disclosed in U.S. Pat. No. 3,674,334. This mirror optical system solves the above-described blocking problem by using part of reflecting mirrors which are rotationally symmetrical about the optical axis.
In the mirror optical system shown in FIG. 38, a concave mirror 111, a convex mirror 113 and a concave mirror 112 are arranged in the order of passage of the light beam, and these mirrors 111, 113 and 112 are reflecting mirrors which are rotationally symmetrical about an optical axis 114, as shown by two-dot chain lines in FIG. 38. In the shown mirror optical system, a principal ray 116 of an object light beam 115 is separated from the optical axis 114 to prevent shading of the object light beam 115, by using only the upper portion of the concave mirror 111 which is above the optical axis 114 as viewed in FIG. 38, only the lower portion of the convex mirror 113 which is below the optical axis 114 as viewed in FIG. 38, and only the lower portion of the concave mirror 112 which is below the optical axis 114 as viewed in FIG. 38.
FIG. 39 is a schematic view of the mirror optical system disclosed in U.S. Pat. No. 5,063,586. The shown mirror optical system solves the above-described problem by decentering the central axis of each reflecting mirror from an optical axis and separating the principal ray of an object light beam from the optical axis. As shown in FIG. 39 in which an axis perpendicular to an object plane 121 is defined as an optical axis 127, a convex mirror 122, a concave mirror 123, a convex mirror 124 and a concave mirror 125 are arranged in the order of passage of the light beam, and the central coordinates and central axes 122a, 123a, 124a and 125a (axes which respectively connect the centers of reflecting surfaces and the centers of curvature thereof) of the reflecting surfaces of the respective mirrors 122 to 125 are decentered from the optical axis 127. In the shown mirror optical system, by appropriately setting the amount of decentering and the radius of curvature of each of the surfaces, each of the reflecting mirrors is prevented from shading an object light beam 128, so that an object image is efficiently formed on an image plane 126.
In addition, U.S. Pat. Nos. 4,737,021 and 4,265,510 also disclose an arrangement for preventing the shading problem by using part of a reflecting mirror which is rotationally symmetrical about an optical axis, or an arrangement for preventing the shading problem by decentering the central axis of the reflecting mirror from the optical axis.
One example of a catadioptric optical system which uses both a reflecting mirror and a refracting lens and has a magnification varying function is a deep-sky telescope such as that disclosed in each of U.S. Pat. Nos. 4,477,156 and 4,571,036. The deep-sky telescope uses a parabolic reflecting mirror as a primary mirror and has a magnification varying function using an Erfle eyepiece.
Another variable magnification optical system is known which varies the image forming magnification (focal length) of a photographing optical system by relatively moving a plurality of reflecting mirrors which constitute part of the aforesaid type of mirror optical system.
For example, U.S. Pat. No. 4,812,030 discloses an art for performing variation of the magnification of the photographing optical system by relatively varying the distance between the concave mirror 101 and the convex mirror 102 and the distance between the convex mirror 102 and the image plane 103 in the construction of the Cassegrainian reflecting telescope shown in FIG. 37.
FIG. 40 is a schematic view of another embodiment disclosed in U.S. Pat. No. 4,812,030. In the shown embodiment, an object light beam 138 from an object is made incident on and reflected by a first concave mirror 131, and travels toward an object side as a converging light beam and is made incident on a first convex mirror 132. The light beam is reflected toward an image forming plane by the first convex mirror 132 and is made incident on a second convex mirror 134 as an approximately parallel light beam. The light beam is reflected by the second convex mirror 134 and is made incident on a second concave mirror 135 as a diverging light beam. The light beam is reflected by the second concave mirror 135 as a converging light beam and forms an image of the object on an image plane 137. In this arrangement, by varying the distance between the first concave mirror 131 and the first convex mirror 132 and the distance between the second convex mirror 134 and the second concave mirror 135, zooming is performed and the focal length of the entire mirror optical system is varied.
In the arrangement disclosed in U.S. Pat. No. 4,993,818, an image formed by the Cassegrainian reflecting telescope shown in FIG. 37 is secondarily formed by another mirror optical system provided in a rear stage, and the magnification of the entire photographing optical system is varied by varying the image forming magnification of that secondary image forming mirror optical system.
In any of the above-described reflecting types of photographing optical systems, a large number of constituent components are needed and individual optical components need to be assembled with high accuracy to obtain the required optical performance. Particularly since the relative position accuracy of each of the reflecting mirrors is strict, it is indispensable to adjust the position and the angle of each of the reflecting mirrors.
One proposed approach to solving this problem is to eliminate the incorporation error of optical components which occurs during assembly, as by forming a mirror system as one block.
A conventional example in which a multiplicity of reflecting surfaces are formed as one block is an optical prism, such as a pentagonal roof prism or a Porro prism, which is used in, for example, a viewfinder optical system. In the case of such a prism, since a plurality of reflecting surfaces are integrally formed, the relative positional relationships between the respective reflecting surfaces are set with high accuracy, so that adjustment of the relative positions between the respective reflecting surfaces is not needed. Incidentally, the primary function of the prism is to reverse an image by varying the direction in which a ray travels, and each of the reflecting surfaces consists of a plane surface.
Another type of optical system, such as a prism having reflecting surfaces with curvatures, is also known.
FIG. 41 is a schematic view of the essential portion of the observing optical system which is disclosed in U.S. Pat. No. 4,775,217. This observing optical system is an optical system which not only allows an observer to observe a scene of the outside but also allows the observer to observe a display image displayed on an information display part, in the form of an image which overlaps the scene.
In this observing optical system, a display light beam 145 which exits from the display image displayed on an information display part 141 is reflected by a surface 142 and travels toward an object side and is made incident on a half-mirror surface 143 consisting of a concave surface. After having been reflected by the half-mirror surface 143, the display light beam 145 is formed into an approximately parallel light beam by the refractive power of the half-mirror surface 143. This approximately parallel light beam is refracted by and passes through a surface 142, and forms a magnified virtual image of the display image and enters a pupil 144 of an observer so that the observer recognizes the display image.
In the meantime, an object light beam 146 from an object is incidence on a surface 147 which is approximately parallel to the reflecting surface 142, and is then refracted by the surface 147 and reaches the half-mirror surface 143 which is a concave surface. Since the concave surface 143 is coated with an evaporated semi-transparent film, part of the object light beam 146 passes through the concave surface 143, is refracted by and passes through the surface 142, and enters the pupil 144 of the observer. Thus, the observer can visually recognize the display image as an image which overlaps the scene of the outside.
FIG. 42 is a schematic view of the essential portion of the observing optical system disclosed in Japanese Laid-Open Patent Application No. Hei 2-297516. This observing optical system is also an optical system which not only allows an observer to observe a scene of the outside but also allows the observer to observe a display image displayed on an information display part, as an image which overlaps the scene.
In this observing optical system, a display light beam 154 which exits from an information display part 150 passes through a plane surface 157 which constitutes part of a prism Pa, and is made incident on a parabolic reflecting surface 151. The display light beam 154 is reflected by the reflecting surface 151 as a converging light beam, and forms an image on a focal plane 156. At this time, the display light beam 154 reflected by the reflecting surface 151 reaches the focal plane 156 while being totally reflected between two parallel plane surfaces 157 and 158 which constitute part of the prism Pa. Thus, the thinning of the entire optical system is achieved.
Then, the display light beam 154 which exits from the focal plane 156 as a diverging light beam is totally reflected between the plane surface 157 and the plane surface 158, and is made incident on a half-mirror surface 152 which consists of a parabolic surface. The display light beam 154 is reflected by the half-mirror surface 152 and, at the same time, not only is a magnified virtual image of a display image formed but also the display light beam 154 is formed into an approximately parallel light beam by the refractive power of the half-mirror surface 152. The obtained light beam passes through the surface 157 and enters a pupil 153 of the observer, so that the observer can recognize the display image.
In the meantime, an object light beam 155 from the outside passes through a surface 158b which constitutes part of a prism Pb, then through the half-mirror surface 152 which consists of a parabolic surface, then through the surface 157, and is then made incident on the pupil 153 of the observer. Thus, the observer visually recognizes the display image as an image which overlaps the scene of the outside.
As another example which uses an optical element on a reflecting surface of a prism, optical heads for optical pickups are disclosed in, for example, Japanese Laid-Open Patent Application Nos. Hei 5-12704 and Hei 6-139612. In these optical heads, after the light outputted from a semiconductor laser has been reflected by a Fresnel surface or a hologram surface, the reflected light is focused on a surface of a disk and the light reflected from the disk is conducted to a detector.
However, in any of the aforesaid optical systems composed of conventional refracting optical elements only, a stop is disposed in the inside of the optical system, and an entrance pupil is in many cases formed at a position deep in the optical system. This leads to the problem that the larger the distance to a pupil plane lying at a position which is the closest to the object side as viewed from the stop, the effective ray diameter of the entrance pupil becomes the larger with the enlargement of the angle of view.
In any of the above-described mirror optical systems having the decentered mirrors, which are disclosed in U.S. Pat. Nos. 3,674,334, 5,063,586 and 4,265,510, since the individual reflecting mirrors are disposed with different amounts of decentering, the mounting structure of each of the reflecting mirrors is very complicated and the mounting accuracy of the reflecting mirrors is very difficult to ensure.
In either of the above-described photographing optical systems having the magnification varying functions, which are disclosed in U.S. Pat. Nos. 4,812,030 and 4,993,818, since a large number of constituent components, such as a reflecting mirror or an image forming lens, are needed, it is necessary to assemble each optical part with high accuracy to realize the required optical performance.
In particular, since the relative position accuracy of the reflecting mirrors is strict, it is necessary to adjust the position and the angle of each of the reflecting mirrors.
As is known, conventional reflecting types of photographing optical systems have constructions which are suited to a so-called telephoto lens using an optical system having an entire large length and a small angle of view. However, if a photographing optical system which needs fields of view from a standard angle of view to a wide angle of view is to be obtained, the number of reflecting surfaces which are required for aberration correction must be increased, so that a far higher component accuracy and assembly accuracy are needed and the cost and the entire size of the optical system tend to increase.
The above-described observing optical system disclosed in U.S. Pat. No. 4,775,217 is realized as a small-sized observing optical system which is composed of a plane refracting surface and a concave half-mirror surface. However, the exit surface 142 for the respective light beams 145 and 146 from the information display part 141 and the outside needs to be used as a total reflecting surface for the light beam 145 exiting from the information display part 141, so that it is difficult to give a curvature to the surface 142 and no aberration correction is effected at the exit surface 142
The above-described observing optical system disclosed in Japanese Laid-Open Patent Application No. Hei 2-297516 is realized as a small-sized observing optical system which is composed of a plane refracting surface, a parabolic reflecting surface and a half-mirror consisting of a parabolic surface. In this observing optical system, the entrance surface 158 and the exit surface 157 for the object light beam 155 from the outside are formed to extend so that their respective extending surfaces can be used as total reflecting surfaces for guiding the light beam 154 which exits from the information display part 150. For this reason, it is difficult to give curvatures to the respective surfaces 158 and 157 and no aberration correction is effected at either of the entrance surface 158 and the exit surface 157
The range of applications of either of the optical systems for optical pickups which are disclosed in, for example, Japanese Laid-Open Patent Application Nos. Hei 5-12704 and Hei 6-139612 is limited to the field of a detecting optical system, and neither of them satisfies the image forming performance required for, particularly, an image pickup apparatus which uses an area type of image pickup device, such as a CCD.